Organometallic electrolyte for galvanic deposition of zinc, aluminum, gallium and indium



United States Patent 3,418,216 ORGANOMETALLIC ELECTROLYTE FOR GAL- VANTC DEPOSITION 0F ZINC, ALUMINUM, GALLIUM AND INDIUM Richard Dotzer, Nuremberg, Germany, assignor to Siemens Aktiengesellschaft, a corporation of Germany No Drawing. Filed Dec. 16, 1965, Ser. No. 515,804 Claims priority, application Germany, Dec. 17, 1964, S 94,664 7 Claims. (Cl. 20 414) ABSTRACT OF THE DISCLOSURE Described is an organometallic electrolyte for galvanic depositing of a metal selected from the group consisting of zinc, aluminum, gallium and indium consisting of a complex compound between an organometallic compound of the formula MeX R and a quaternary onium salt of the formula [R R R R Y]X, whereby Me is a zinc, aluminum, gallium or indium atom, X a halogenide, pseudohalogenide or half a sulphate radical, R an alkyl radical with C to C Y a nitrogen, phosphorus, arsenic or tellurium atom, and R R R R represent hydrocarbons and m the values 0, 1, or 2, n the values 1, 2 or 3 and m-l-n is equal to the valence of the Me-atom, wherein at least one of the hydrocarbon radicals R to R is selected from the group of benzyl, phenyl, cyclohexyl and strongly branched hydrocarbon radical with C to C and the remaining radicals of the onium salt are C to C alkyls.

My invention relates to an organometallic electrolyte for galvanic depositing of the metals zinc, aluminum, gallium and indium.

There is technological great interest in effective and easy handling of electrolytes for galvanic depositing of zinc, aluminum, gallium and indium. Aluminum, indium and zinc are, for example, good contacting materials which are widely used, particularly in semiconductor technology. Galvanic depositing may contribute to the production of layers of intermetallic, superconducting compounds of gallium. Aluminum and zinc are also very suitable for electroplating, especially aluminum which can absolutely not be deposited from aqueous solutions. Aluminum is of great technical interest because of its ability to assume a colored surface.

Organometallic electrolytes for galvanic deposition of aluminum are already known. These are complex compounds of aluminum alkyls and alkali halogenides as well as homogeneous liquid phase aluminum alkyls and their complex combinations with quaternary ammonia compounds.

The known organometallic electrolytes for depositing aluminum have the disadvantage that they are self-igniting in the open air and that they react with water very violently and, often, accompanied by flames. The selfignition in air may be traced back to the self-ignition capacity (auto-inflammability) of the metal alkyls present in the electrolytes. The above described disadvantages greatly reduce the technological applicability in a wider, practical sense, for example, for the purpose of aluminum plating, although the possibility exists to reduce the infiammability of the electrolyte by adding benzenetoluene, or xylene. This entails, however, the risky handling both of self-igniting electrolytes and flammable hydrocarbon. In addition, after an initial slight reduction in conductivity, the addition of the hydrocarbons sharply reduces the conductivity of the thinned electrolyte systems with an increased content of hydrocarbon.

For these reasons it is an object of this invention to develop electrolytes, for galvanic depositing of aluminum, which can be handled without risk. Along with this ob- 3,418,216 Patented Dec. 24, 1968 ject, where preferred, organometallic electrolytes were developed for the purpose of galvanic deposition of aluminum. Another object is to develop electrolytes for deposition of zinc, gallium and indium. These constitute the present invention.

The invention concerns an organometallic electrolyte for galvanic depositing of one of the elements zinc, aluminum, gallium or indium, consisting of a complex compound between a metal compound of the formula MeX R and a quaternary onium salt of the formula [R R R R Y]X whereby:

Me is an atom of zinc, aluminum, gallium or indium,

X a halogenide, pseudohalogenide or half of a sulphate,

R is an alkyl radical with C to C Y is a nitrogen, phosphorus, arsenic or tellurium atom,

and

R R R R means hydrocarbon radicals and m are values 0, 1 or 2,

n are values 1, 2 or 3 and m-l-n is equal to the capacity of the element to combine with hydrogen.

According to the invention, the electrolyte is characterized by the fact that at least one of the hydrotarbor radicals R to R is a benzyl, phenyl, cyclohexyl or a strongly branched hydrocarbon radical with C to C while the remaining radicals of the onium salt are alkyl radicals with C to C The molar ratio between onium salt and the metal alkyl in the complex compound amounts, in the electrolyte according to the invention, is 1:1 or preferably, 1:2.

It was surprisingly possible, through inserting of benzyl, phenyl or cyclohexyl radicals of strongly branched hydrocarbon radicals into the onium ion of the complex compounds, to reduce the inclination toward violent autoxidation and hydrolysis of the organometallic compounds of zinc, aluminum, gallium and indium, to such an extent that the electrolytes, according to the invention, lose their feared self-igniting tendency and the explosive violent hydrolysis and the electrolyte can be handled virtually danger free in air.

It is of importance thereby that the effective organic groups, i.e. the benzyl, phenyl, cyclohexyl or branched hydrocarbon radicals are inserted into the onium ion and not into the metal alkyl molecule of the electrolyte. While the variably large organic groups are tightly bound to the central atom of the onium ions, i.e. to the nitrogen, phosphorus, arsenic or tellurium atom and do not undergo any exchange or disproportioning process, the mixed organometals of zinc, aluminum, gallium and indium are not stable, but disproportionate into the pure orgauometals. For example, three moles of the non-stable benzyl diethyl aluminum would disproportionate (rearrange) into two moles of the stable triethyl aluminum and one mole of the stable tribenzyl aluminum. Metal organic electrolyte complexes of organometals, which contain 'various organic radicals and onium salts cannot be produced due to the instability of the mixed organometals.

The self-ignition danger and hydrolysis tendency is especially sharply reduced in electrolytes according to the invention, which consist of a complex compound of a mole of an onium salt and one or two moles of a metal alkyl, and whose onium salt contains the benzyl group C H CH Thus, for example, autooxidation and the rate of hydrolysis in the electrolyte combination, which is very useful in galvano technology, trimethylbenzylammoniumhexethylfluorodialanate 3) 3 e s z z s) 3 2 a] respectively s) 3( e 5 2) 2 5) s is reduced so much that the compound which is liquid at room temperature does not even emit smoke when contacting air and no longer burns the skin. Hence, a dangerfree handling of this electrolyte, for the purpose of galvanic depositing, is therefore quite possible. The same applies to the corresponding chloro compound.

The zinc alkyls, which are even more inflammable by air than the aluminum alkyls, become, in complexes with onium salts containing benzyl groups, readily manageable electrolyte liquids, which may be used for depositing zinc.

Thus, for example, the 1:1 complex compound and the corresponding 1:2 complex compound are also liquid and not self-igniting at room temperature. They emit only a slight smoke in the air and their reaction with water has only slight vehemence. They are good organometallic electrolytes for depositing zinc, having specific conductances of 2.4 resp. 2.1- ohm -cm. at 100 C.

In the table which follows various 1:1 and 1:2 complex compounds of metal alkyls and onium salts containing groups of benzyls are listed, and are suitable as electrolytes for galvanic deposition of zinc, aluminum, gallium and indium. The table also indicates the temperature range wherein the complex compounds melt. Also listed are the specific conductances at 100 C.

The melting ranges of the complex compounds listed in the table are below 0 C. :with the exception of the trimethyl indium complex mentioned at the bottom. Hence, these complex compounds are liquid at room temperature. The trimethyl indium complex compound also liquefies at slightly higher temperatures. The relatively high specific conductivities at 100 C. makes these complex compounds good organometallic electrolytes. They may be produced according to the method disclosed in my US. patent application Ser. No. 355,222.

The phenyl radicals C H and the cyclohexyl radicals C H due to their favorable space filling structure, when inserted into the onium ion, provide surprisingly good shielding effects against the penetration of air or moisture into the complex compounded metal al'kyl. The melting ranges of those electrolyte compounds, which contain phenyl and cyclohexyl groups, are not as low as the melting ranges of the electrolyte complex compounds containing a benzyl group in the onium ion.

Hence, the phenyl and cyclohexyl groups reduce the melting point of the complex compound to a lesser degree, however they are also liquid at room temperature.

An example of a phenyl containing electrolyte is the complex compound trimethylphenylammonium-tricthylchloroalanate,

The melting range of this compound is 2022 C. The compound has an electric conductance of 149-10 ohm cm.- The 1:2 complex correspond thereto has a melting scope of 97 to --l00 C. and a specific conductance of 1.81-10 ohm cm.- and is therefore a very suitable electrolyte for depositing aluminum.

Both compounds may be very favorably produced according to the method disclosed in application Ser. No. 355,222 by reaction of readily produced components dimethylaniline, chloromethane and aluminumtriethyl.

Completely analogous to the above, by adding 1,2- dichloorethane to triphenylphosphine and aluminumtriethyl, the quaternary phosphonium salt complex compound may be obtained This electrolyte complex first solidifies below 33 C., and at 100 C. has a specific conductance capacity of 2410* ohmcm. In this compound, a hydrogen atom of the ethyl radical in the phosphonium ion is substituted by a chlorine atom, which reduces flammability even further.

Since halogenide contents in organic compounds generaly reduce the flammability of the latter, it is possible, according to the invention, to use, for example, instead of benzyl chloride in onium ions of the complex compounds listed in Table I, the commercially sold p-chlorinebenzyl-chloride for the purpose :of producing the electrolyte according to the invention. The same applies to pchlorodimethylaniline.

Other organic groups which also reduce the autooxidation and the hydrolysis rate of the complex compounds, are the strongly branched, and therefore space filling, hydrocarbon radicals with C to C for example, the tertiarybutyl radical-C(CH and the isopropyl radi- Cai'CH(cH3)2.

An example of an electrolyte complex, containing such radicals is diethylisopropyltelluriumdiethyldibromogallanate which is suitable for electrolytic deposition of gallium. This compound is oily at room temperature and does not smoke in the air and reacts with water slowly and without emitting flames. It may be easily produced by combining diethylbromidegallium, diethyltellurium and isopropylbromide. At 100 C., it has a specific conductance of 1.96-10 ohmcm? and solidifies only below -74 C.

It should be pointed out that it is possible to insert from 1 to 4 organic radicals into the onium ion of the complex compound, which according to the invention, react favorably. However, it is preferable to use, for electrolytes according to the invention, only those complex compounds whose onium ion contains only one or two of the radicals listed as eifective according to the invention. In this group are hydrocarbon radicals, which due to their size, eflFect a reduction of the electrical conductance of the complex compound into whose onium ion they are inserted. The insertion of only one or two of these radicals surprisingly yields a reduction in the autooxidation and hydrolysis rate of the complex compound, which is quite adequate for practical purposes, while the reduction of the conductance capacity does not create too great difliculty. As a rule, however, the insertion of the radicals, according to my invention, very considerably lowers the melting point of the complex compounds.

This factor is very essential in the handling of the quantity and the temperature of the electrolyte bath. The

solubility of the complexes containing an inserted radical, is increased in aromatic hydrocarbons. This is very important for the washing processes which, for galvanotechnical reasons, follow electroplating of objects.

The following table illustrates how the insertion of a phenyl, respectively, a benzyl radical, in place of a methyl radical in the onium ion of an aluminum complex compound efiects a reduction in the melting range and the specific conductance. However, while the aluminum compound, whose onium ion contains four methyl groups, autooxidizes and has a strong tendency toward hydrolysis, the two other compounds named in the table, are favorable electrolytes, according to the invention.

The loss of conductance capacity may be partially compensated by thinning the complex compound with 2-4 mols of aromatic hydrocarbon, for example, benzene, toluene or xylene, or higher aliphatic or cyclic ethers, for example, di-n-butylether, isopropylether, tetrahydrofuran or dioxane.

In the above listed alcohols and ethers, the electrolyte complex compounds strongly dissociate into onium and metalalkylhalogenide complexes. This increases somewhat the conductance capacity of the electrolyte liquid.

According to a further aspect of my invention, particularly suitable electrolytes with little flammability are obtained through a combination of onium ions, which contain benzyl, phenyl, cyclohexyl or strongly branched hydrocarbon radicals, with higher halogen-substituted organometal ions, into a complex compound.

The electrolytes, which are comprised of a complex compound of an onium salt containing the above mentioned hydrocarbon radicals and a metal alkyl containing one or two halogen substituents, are distinguished by low flammability, poor self-ignition, slight tendency toward autooxidation and hydrolysis, a relatively low melting temperature and a good electrical conductance. These electrolytes are therefore extraordinarily suitable for galvanic depositing of the metals zinc, aluminum, gallium and indium.

An example for these electrolytes are the complex compounds which are derived from the trimethylbenzylammonium-fluoride-aluminum-triethyl complex by substituting fluorine for one or two ethyl radicals. In the following Table III, the melting range and the electric conductivity of these compounds are compared to the triethyl aluminum complex compound.

The production of these complex compounds with many times halogenide-substituted metal alkyls takes place, according to a further development of the invention, by reaction of the mol of an acid quaternary onium salt with one or two moles of a metal alkyl.

The acid salts of the quaternary onium salts, which are used as the starting materials for producing the electrolytes according to the present invention, may be produced according to a process which is closely disclosed in 11.8. patent application Ser. No. 367,578. According to this process, the acid salt trimethylbenzylammoniuzmhydrogendifluoride is produced by reacting trimethylbenzylammoniumchloride with an excess of hydrogen fluoride. The acid salt resulting from this reaction is then ether extracted whereby trimethylbenzylammoniumtrihydrogentetrafiuoride occurs. This compound is reacted with sodium alcoholate to yield the colorless, crystalline and hygroscopic compound trimethylbenzylammoniumhydrogendifluoride. The equations for the above described process are:

l(CH:)a(CeHsCHz)N]O1 HF(i.i I.)

To produce the aluminum complex compound trimethylbenzylammonium-diethyl-difluorinealanate (see Table HI), according to the invention, the acid salt trimethylbenzylammonium-hydrogendifluoride is reacted with triethyl aluminum.

The chemical reaction takes place according to the equation:

n-Hexane [(CH3)3(GGH5CH2)N]F.HF Al(C2H5)3 The following example discloses the production of this electrolyte in still greater detail.

47.4 g. trimethylbenzylammonium-hydrogendifluoride (0.25 mole) were suspended in 100 ml-n-hexane. 28.6 g. triethylaluminum were slowly dripped in, under strong mixing, for minutes. The reaction is very exothermic and with each drop there are ethane gas developments. Gradually, the suspension becomes a pulp and finally one obtains an oily liquid which settles below the hexane. After refluxing the hexane for one hour, the liquid is left to cool. The supernatant hexane is subsequently separated. Possible hexane residues are drawn off, accompanied by slight heating. The reaction product is a clear oily liquid and shows at C. the specific electric conductance capacity of 1.3-10- ohm -cm.

The electrolyte, which solidifies at 15 to -17 C., is thermically stable up to C. and shows no development of smoke or heat in the air. The reaction with water is also slow and without danger. Due to the low autooxidation :and rate of hydrolysis there is no danger of self-ignition. Thus, the electrolyte is harmless and easy to handle.

If an equimolar amount of freshly distilled triethylaluminum is added to this 1:1 complex, one obtains then, according to the formula an asymmetrical electrolyte complex of equally good conductivity and a strongly reduced :autooxidation and hydrolysis rate. At room temperature, this electrolyte is liquid and at 100 C. has a specific conductivity of 1.4-10 ohm -cm.

T o produce this electrolyte (also appearing in Table 111), trimethylbenzylammonium ethyltrifluorinealanate, liquid acid salt trimethylbenzylammonium-dihydrogenfluoride is reacted with triethylaluminum at room temperature, according to the equation:

At a 0.25 molar composition, 52.3 g. of the liquid and almost colorless trimethylbenzylammonium-dihydrogentrifiuoride were deposited under 75 ml. n-hexane. Then 28.6 g. triethylaluminum were dropped in slowly, accompanied by vigorous mixing, at room temperature. Vigorous development of ethane and boiling of the solvent show that a strongly exothermic reaction took place. After adding triethyl aluminum over a period of about 90 minutes, reflux heating was carried out for about 2 hours (boiling temperature of the solvent), until foam formation ceased. During the cooling process and after turning off the mixer, the solvent separates quickly from the heavy electrolyte complex. The latter solidifies at 16 to 18 C. and has a specific conductance of 1.2-1O- ohm -cm.- at 100 C.

If an equimolar amount of triethylaluminum is added to this easiiy handled 1:1 complex electrolyte liquid, then according to the reaction the symmetrical 1:2 complex of the electrolyte system develops. As in all triethylaluminum additions for the purpose of forming the 1:2 complex, here also the second mole triethylaluminum results in an exothermic reaction, which shows the great tendency for forming this 1:2 complex.

By reacting the acid oniumfiuoride with zinc, gallium or indium alkyls, one obtains favorable electrolytes for a galvanic depositing of these metals.

The acid onium chlorides and onium bromides may be reacted in the same manner as the acid oniumfluorides, with the metal alkylys of zinc, aluminum, gallium or indium.

The method, according to the invention, of producing electrolyte complex compounds with higher halogenide substituted metal alkyls of acid onium salts has many advantages. The reactants are easily available. The acid halogenides are even more readily obtainable as solvent free quaternary onium compounds, than the neutral onium salts. Trialkyl-aluminum may be purchased. Trialkylgallium and-indium may be produced according to the methods disclosed in co-assigned applications Ser. Nos. 162,212 and 108,996 now respectively Patent Nos. 3,310,574 and 3,318,931. A method of producing dialkyl zinc is found in the Hiither thesis (Technische Hochschule Aachen, 1957). The rapid and complete conversion of the solid salts is expedited by the gas development, in the present method, i.e. through the escape of alkanes.

The strongly exothermic reactions are preferably performed in ethers, aliphatic or aromatic hydrocarbons, for example, in diethylether, hexane, benzene or toluene. The most favorable reaction mediums were in the aromatic hydrocarbons, since they dissolve the developing complex salts. Furthermore, the solutions of the onium salt complex compound may find use as electrolytes. If the complex compounds are to be obtained per se, then aliphatic ethers or hydrocarbons are recommended as a solvent for removing the considerable heat of reaction.

After the reaction has been completed, the solvents are removed by distillation under reduced pressure. The electrolyte complex compounds are thereby substantially preserved as such.

Several examples of the galvano-technical use of the electrolytes of this invention are described hereinbelow in greater detail:

(a) aluminum electroplating of copper, brass and nickel sheets in the electrolyte of toluene per mole of complex, all heated to 80-100 C. are introduced into a galvanizing vessel 100 x 200 x 200 mm., consisting of glass and having an aluminum cover which carries all electrodes, openings for two mechanical agitators, inert gas inlet, thermometer and an opening for adding the supplement. The sheet metal strips 50 x 200 mm., to be coated with aluminum are, in a known manner, mechanically pre-treated and etched. The strips are passed with acetone from an aqueous into an non-aqueous system and stored, moistened with benzene. They are arranged at a distance of about 20 mm. between purified aluminum anodes (50 x 150 X 10 mm.) and approximately half dipped in the electrolyte. Under severe agitation of the electrolyte, with the aid of mechanical agitators, at a bath voltage of 2 to 3 volts and a cathode current density between 0.2 to 1.0 a./dm. for example, 0.5 a./dm. a 10 to 15 aluminum layer is applied in approximately 3 hours. The sheet metal pieces are removed after the electroplating process and the electrolyte is Washed oif in benzene or toluene. The aluminized surfaces have a dense aluminum coating, which may be very easily polished to a highly reflecting mirror and which is very suitable for producing a colored surface. For this purpose, it is favorable to apply aluminum deposits between 20 and 40, thick.

(b) aluminum electroplating of molybdenum discs in an electrolyte In a nickel cathode disc, 13 recesses of 1 mm. depth have been provided for receiving molybdenum discs of 20 mm. in diameter and 2 mm. thick. A rotating anode with a diameter of 100 mm. and a thickness of 5 mm. is arranged about 15 mm. above the cathode disc which is within a 1 liter mixing vessel made of Jena glass. 200 ml. of unthinned electrolyte are inserted into the dry electrolyte vessel which is filled with argon. Electroplating took place at 100 C. bath temperature, a voltage of 2.5 volts and a current density of 1.0 a./dm. A fine crystalline aluminum layer, about 20a in thickness was deposited over a period of three hours.

A second combination utilized an electrolyte, thinned with an equimolar amount of toluene. The bath temperature was C. The electrical data was 0.5 a./dm. cathode current density at 1.3 to 1.5 v. cell voltage. An aluminum layer of approximately 80a thickness grew in 16 hours and had a fine crystalline, brightly polished (shining)appearance.

In a third combination, an additional thirteen molybdenum discs were electroplated with the same electrolyte, at C. in about 12 hours, with a thin, about 70y. thick, brightly polished medium-fine crystalline aluminum layer. In this test, the cathode current density amounted to 0.5 a./dm. the cell voltage 1.2 volt.

The invention can be carried out other than as specifically described above. The invention is to be limited only by the scope of the appended claims.

I claim:

. 1. An organometallic electrolyte for galvanic depositing of a metal selected from the group consisting of zinc, aluminum, gallium and indium consisting of a complex compound between an organometallic compound of the formula MeX R and a quaternary onium salt of the formula [R R R R Y]X whereby Me is a zinc, aluminum, gallium or indium atom,

X a halogenide, pseudohalogenide or half a sulphate radical,

R an alkyl radical with C to C Y a nitrogen, phosphorus, arsenic or tellurium atom, and

R R R R represent hydrocarbons and m the values n the values 1, 2 or 3 and m+n is equal to the capacity of the element to combine with hydrogen wherein at least one of the hydrocarbon radicals R to R is selected from the group of benzyl, phenyl, cyclohexyl and strongly branched hydrocarbon radical with C to C and the remaining radicals of the oniurn salt are C to C alkyls.

2. The electrolyte of claim 1 wherein the mole ratio between the onium salt and the organometallic compound is 1:1.

3. The electrolyte of claim 1 wherein the mole ratio between the anium salt and the organometallic compound is 1:2.

4. The electrolyte of claim 1 as a melt of the complex compound.

5. The electrolyte of claim 1 dissolved in aromatic hydrocarbons,

6. The electrolyte of claim 1 dissolved in higher aliphatic or in cyclic ethers.

7. The electrolyte of claim 1 wherein in the organometallic compound of the general formula MeX R 5 X is a halogenide and m is an integer from 1 to 2.

References Cited UNITED STATES PATENTS 8/1966 McGraw 20414.1 3/1967 Poe et al. 260-448 HOVARD S. WILLIAMS, Primary Examiner.

T. TUFARIELLO, A ssislant Examiner.

US. Cl. X.R. 106-1 

